1. Field of the Invention
The invention relates to a memory element, and more particularly to a phase-change memory element.
2. Description of the Related Art
A phase-change memory must be highly reliable, have fast speeds, have low current, and have low operating voltage, in order to become a viable alternative to current memories such as flash and DRAM. A phase-change memory cell must therefore provide low programming current, low operating voltage, a small size, and fast phase transformation speeds, at low costs. The requirements are difficult to meet given the current state of the art.
Phase-change memories are mainly used in devices that utilize non-volatile flash memories, such as mobile devices which require low power consumption, and hence, minimal programming currents. Thus, a phase-change memory cell is designed to provide low programming current, high reliability (including electromigration risk), a small cell size, and fast phase transformation speed.
The conventional method to reduce programming current is to reduce the heating area (the contact area between phase-change material layer and electrode) of the phase-change memory. A benefit of the method is simultaneous reduction of cell size and enhancement of integrated density. However, reduction of heating area is limited by the resolution limits of the photolithography process, resulting in minimal reduction of programming current. Further, reducing the heating area of the phase-change memory results in higher cell resistance, which increases required driving voltage. Thus, all other considerations being the same, the amount of Joule heating is conserved, wherein the operating voltage is inversely proportional to the programming current, which is not desirable. Specifically, reducing the heating area does not necessarily improve other performance features of the phase-change memory. Fast phase transformation speed also requires good thermal uniformity within the active regions of the cell.
In reality, cooling becomes significant for smaller structures, and heat loss to surrounding areas becomes more important with increased surface/volume ratio. As a result, temperature uniformity is degraded. In addition, required current density must increase as heating area decreases. Thus, increasing reliability concerns for electromigration. Hence, in practice, it is important to not only reduce the current of the phase-change memory, but also required heating. Specifically, with a decrease in the amount of Joule heating input, heat loss to surrounding areas must be decreased even further.
Kostylev et al. (U.S. Publication No. 20070235709) disclosed a phase-change memory comprising an electrode with a sidewall in contact with a phase-change material layer. The aforementioned structure, however, suffers from low heating efficiency due to current being forced to spread outward.
Lung et al. (U.S. Publication No. 20070215852) disclosed a phase-change memory comprising a pipe-shaped electrode in contact with a phase-change material layer. The aforementioned structure, however, is also apt to result in reduced heating efficiency, as current is forced to spread inward instead of outward as disclosed in Kostylev et al.
Happ et al. (U.S. Publication No. 20070190696) disclosed a phase-change memory comprising a bottom electrode, an isolation layer with an opening formed on the bottom electrode, a barrier layer conformally formed on the isolation layer, a phase-change material layer filled in the opening, and a top electrode formed on the phase-change material layer. The heating area, however, is limited by the diameter of the opening formed by a photolithography process, thereby hindering increase of heating efficiency and reduction of the programming current of the phase-change memory cells.
Therefore, it is desirable to provide a phase-change memory cell structure that improves upon the aforementioned problems.